Telephone-exchange system



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ltheii UNETEB STATEfi PATENT @FFEQEQ FRANKLIN A. STEARN, OF PATERSON, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE-EXCHANGE SYSTEM.

Application filed June 3, 1921. Serial No. 474,702.

To all, whom it may concern:

Beit known that I, FRANKLIN A. STEARN, a citlzen of the United States, residing at Paterson, in the county of Passaic, State of New Jersey, have invented certain new and useful Improvements in Telephone- Exchange Systems, of which the following is a full, clear, concise, and exact description.

This invention relates to telephone exchange systems in'general and especially to those systems wherein machine switching apparatus is employed in the central oflices for the extension of conversational connections.

Districts embracing extensive and thickly populated areas, such as the territory of the larger cities, must be provided, in order to furnish adequate telephonic communication, with an exchange system consisting of a large number of individual central offices suitably distributed throughout the entire area. The close relation that exists between the various parts or localities, served by their own individual central offices, which collectively make up the exchange area, de-

mands a communicating system in which quick and efficient service may be had in the interconnection of lines belonging to any two oflices. To accomplish this universal service by providing interoffice trunk groups between each Office and every other oflice in the area is highly objectionable. The cost of installing and maintaining so intricate a trunk network in congested city districts is prohibitive.

To obviate the expense incident to the provision of interofiice trunk groups for all offices in the system, and at the same time to give every office facilities for communicating with every other office, the tandem switching basis is adopted as a satisfactory solution of the problem of furnishing eliicient and economical service in communities of the character above mentioned. According to the tandem plan a central switching office is selected at some convenient position in the area it serves and from this tandem point groups of trunks radiate to all of the outlying ofiices. Any office wishing to establish a connection with any other oflice in the system may do so by routing the call first to the tandem switching point from whence it is directed to the desired terminat ing oflice. The economy in trunks made possible by this method is at once obvious.

WVhere all oflices in thesystem, both originating and terminating are of the manual ,type the requirements placed upon the central tandem ofiice are relatively simple. Operators located at thls oflice recelve, over order wires, their instructions from the operators in the originating ofiice. They then proceed to connect the assigned trunk incoming from the originating office to the proper outgoing trunk to the called office. In the called office the connection may be completed to the desired line by the operator thereat. But the exchange systems to be considered hereinafter are those which have been converted partiall from-the manual basis to the machine switching basis. In other words,

complete connections between subscribers lines in such a system are effected in some cases partly by manual operators equipment and partly by machine switches, while in other instances the completion of connections may be entirely automatic. Certain offices may be provided with manual operators. positions and equipment, other offices with full mechanical apparatus and still other offices in the area equipped with call indicating mechanism for communicating to operators the designations of called lines. Consequently, the central tandem switching office must be adapted to perform the necessary cooperation with these various kinds of outlying offices.

To this end the tandem office is equipped with automatic selective switches, a district and an oflice switch. The trunks incoming from originating oflices terminate in the district switches, which, together with the'ofiice selectors, are operable in accordance with the designation to extend the incoming trunk over a trunk outgoing from the tandem point to the called-ofiice. The control of the district and office switches, to select trunks extending to any of the large number of outlying offices, is exercised by register senders located at the tandem office. These senders are also adapted to control the apparatus in the called office, whether such apparatus be automatic switches or call indicating mechanism. The complete record of a call designation, therefore, is comprised of two or more distinct parts. One is the code which identifies the particular oflice for which the call is destined and governs the operation of the district and oflice switches at the tandem point to select a trunk lead- 

